Method for treating condensates form polycodensates

ABSTRACT

The invention relates to a method for treating vapours and condensates which arise during the production of polycondensates from bisphenols or high-value phenols by esterification and/or transesterification with alkyl or aryl esters of at least bivalent organic or inorganic acids. According to the invention, the treatment is carried out in a plurality of condensates and/or distillation columns which are connected in a step-by-step manner behind each other, each distillation column comprising a connected condensate. The dew point and the pressure in each condensate are adjusted such that in each step, monomers, oligomers or decomposition and transformation products are removed.

The invention relates to a method of treating vapors and condensatesthat are produced during the production of polycondensates frombisphenols or polyhydric phenols by esterification and/ortransesterification with alkyl and/or aryl esters of at least divalentorganic or inorganic acids.

It is known that polycarbonates, polyarylates, and copolymers ofpolyethylene terephthalate, polypropylene terephthalate, or polybutyleneterephthalate can be prepared by esterification and/ortransesterification of alkyl or aryl esters of organic or inorganic, atleast divalent acids with bisphenols and subsequent interfacialpolycondensation or melt polycondensation. These polycondensates areknown as engineering plastics for their outstanding properties. Thewater or other vapors released during condensation contain, apart fromthe main cleavage product from the transesterification or esterificationthat is preferably phenol, additional monomers, oligomers, or productsthat are produced through thermal decomposition or rearrangements. Thedecomposition or rearrangement products formed in the reactioncontaminate the monomers and oligomers also still present in the vaporsand make it impossible to return the monomers and oligomers unpurifiedto the polycondensation process when products that are not discoloredand that do not sufficiently meet rheological and mechanical qualityrequirements are to be produced. The cleavage products produced by thepolycondensation, such as phenols, alcohols, and water, are also socontaminated by the mentioned decomposition and rearrangement productsthat they cannot be easily reused. The reaction products arising duringthermal decomposition interfere with the reutilization of phenolspresent in the vapors, for example, for the production of bisphenol A,diphenyl carbonate, triphenyl borate, or for any other phenyl ester ofan organic or inorganic acid.

For this reason, the object arose to develop a method of treating vaporsand condensates that are produced during the production of theabove-described polycondensates, the method making it possible, on theone hand, to recover the entrained monomers and to return them to thepolycondensation and, on the other, to produce the cleavage productsarising in the polycondensation, primarily phenol, in such a pure formthat they can be reused for other reactions without deterioration in theproduct quality.

A method has now been found for treating vapors and condensates that areproduced during the production of polycondensates from bisphenols orpolyhydric phenols by esterification or transesterification with alkylor aryl esters of at least divalent organic or inorganic acids, themethod in which the treatment occurs in several succeeding cascadedcondensers and/or distillation columns with respective connectedcondensers, where in each condenser the dew point and pressure areestablished such that in the individual stages the specific monomers,oligomers, or the decomposition and rearrangement products areseparated.

This method enables the return of useful materials to the process, isespecially economical and cost-effective, avoids environmental pollutionby chemicals, and utilizes the inevitably forming by-products togenerate energy. This method can be used especially well for theregeneration of phenol-containing vapors and condensates and for therecovery of monomers, as they arise, for example, in the production ofpolycarbonates, polyarylates, or in the melt-phase polycondensation ofpolymers and copolymers, such as polyethylene terephthalate,polypropylene terephthalate, or polybutylene terephthalate withdiphenols and bisphenols or polyhydric phenols by esterification ortransesterification with alkyl or aryl esters of organic or inorganic,at least divalent acids and/or the acids themselves.

The above-described polycondensates are known as engineering plasticswith outstanding properties and special fields of application. Theirproduction occurs either by interfacial polycondensation as in the caseof polycarbonate or by means of melt polycondensation in the directpolycondensation method from dicarboxylic acids or dialcohols ordiphenols or by transesterification processes from the correspondingacid esters. In the melt condensation for the production ofpolycarbonates and polyarylates, aromatic dihydroxy compounds, forexample, bis(4-hydroxyphenyl)alkanes, particularly bisphenol A, aretransesterified with diphenyl carbonate or terephthalic diphenolate inthe presence of catalysts with cleavage of phenols, oligomerized, andfinally polymerized in multiple stages under a progressive vacuum.Methods of this type are described in the German patent publicationsDE-B-1 495 730 [U.S. Pat. No. 3,535,280] and DE-C-2 334 852 [U.S. Pat.No. 3,888,826]. In addition, the international patent application WO2002/044244 [U.S. Pat. No. 6,838,543] describes a method by whichpolycarbonates are prepared by reaction of a monomeric carbonatecomponent with at least one diphenol or dialcohol in the presence of atransesterification catalyst; here, the melted components are mixed withthe transesterification catalyst and a product is produced that ispolycondensed. For the polycondensation, the transesterification productis passed through a preliminary reactor, at least one intermediatereactor, and a final reactor, the reactors being connected in series andhaving a substantially horizontally driven shaft with mixing elementsattached thereto. Care is taken that a melt residence time of 5 minutesto 2 hours is maintained in the preliminary reactor and final reactor,the temperatures in the preliminary reactor are maintained within therange of from 220 to 300° C., and the pressure in the preliminaryreactor is maintained in the range of from 100 to 800 mbar and in thefinal reactor in the range of from 0.1 to 50 mbar.

If the operating conditions and process stages described in WO2002/044244 are now applied to the production of polycarbonates,polyarylates, and other polymers or copolymers that were prepared frombisphenol A or other polyhydric phenols with at least divalent acids orthe phenol-containing esters thereof, then different phenol-containingvapor and condensate compositions form depending on the residence time,catalyst, pressure, and temperature. Thus, for example, in a first stageprimarily cleavage products from the esterification ortransesterification arise such as water, phenols, or alcohols that stillcontain minor amounts of other monomers forming the polymer. Apart fromthe conventional cleavage products, depending on the selected reactionconditions, rearrangements and secondary reactions of the monomercompounds can also occur in this case, primarily of the polyhydricphenols and especially the bisphenols.

Thus, for example, it is known from U.S. Pat. No. 4,294,994 that duringtreatment of bisphenol A with acids, formation of isopropenylphenol andits polymers, dihydroxyindanes, dihydroxyspirobisindanes,alkyldistilbestrols, trishydroxyphenyls, and polyhydroxyaryls occurs.These by-products because of their high molecular weight have highmelting and boiling points and escape only under drastic reactionconditions, i.e. at high temperatures and low pressure. They cause,inter alia, discoloration and crosslinking in the polycondensate anddisrupt the molecular structure. Other substances, such as, for examplethe monomers, also have high boiling points, but in the case of the rawmaterials employed for the production of engineering plastics these arefar below those of the above-described secondary reaction products, sothat they can be separated by fractional methods both from the highboilers and from the low boilers.

The method of the invention will now be described in greater detailusing the example of the reaction of diphenyl carbonate with bisphenol Ain the presence of a catalyst, known for this reaction, according toFIG. 1 and FIG. 2.

The monomers, diphenyl carbonate 1 and bisphenol A 2, introduced into areactor 4, are reacted in the presence of a catalyst 3, so that cleavageproducts 5, primarily phenol, accumulate in a large amount at thebeginning of the reaction even at a low temperature and high pressure.The distillate forming hereby is conveyed to a condenser 6 whosetemperature is kept above the dew point of the cleavage products andthat of monomers 1 and 2, for example at a temperature above 200° C. andat 400 mbar. In this way, high-boiling products 7 of the secondaryreactions and present in vapors 5 of the first stage, such asspiroidanes and indanes, can be separated out. The vapors continuingonward then pass through a rectification column 8 in which the lowestboiling cleavage products are stripped off at the top, but monomers 1and 2 are drained off at different trays and returned to the firstreaction stage 4.

The product collected in the bottom of column 8, with high boilers 30,which correspond substantially to the products of the secondaryreaction, is combined with the bottom product 7 from the firstcondensation [6] and conveyed to a collection tank 24.

The low-boiling cleavage product 10 is condensed in a condenser 11 andtaken to a collection tank 28. The first compressor stage 12 that, asindicated in FIG. 1, may consist of a mechanical blower 12, but also ajet exhaust, serves for generating the necessary low pressure. Thecondensates accumulating in the compression stage 12 in condenser 29proceed to a collection tank 27.

In the second reaction section (reactor 14, supplied with product 13from reactor 4), the amount of low-boiling cleavage products 15 is evenlower. In this stage 14, however, because of the higher productviscosity, more drastic measures, such as a higher temperature and lowerpressure than in stage 1, are necessary to drive the reaction or thechain growth forward, thus, for example, 270 to 300° C. and 100 hPa to10 hPa. Because of the higher thermal loading, the products fromsecondary reactions appear increasingly in the vapor gas stream 16; as aresult, the amount of this product type, accumulating in condenser 6′,is also greater than in previous stage 4. The condensate 7, accumulatingin 6′, proceeds to a collection tank 24. The amount of monomers 1 and 2in the cleavage products has become so low that a separation is nolonger worthwhile and everything is condensed in condenser 8′, beforethe lowest-boiling vapors are compressed in a compressor 16. The vaporscondensed in condenser 17 from the compressor 16 are combined with thestream 10 and taken to the collection tank 28. Condensate 9 is suppliedto the rectification stage 8 to increase the yield of monomers 1 and 2and the cleavage products 10.

In the third reaction section (reactor 19, supplied with product 18 fromreactor 14), the lowest amount of cleavage products 20 accumulates.Here, however, because of the highest product viscosity, the mostdrastic measures, such as the highest temperature and lowest pressure,are necessary to drive the reaction or the chain growth forward, thus,for example, 280 to 350° C. and 10 hPa to 0.1 hPa. Because of the highthermal loading, the products in the secondary reaction occupy a largeportion in the vapor gas stream 20; as a result, the amount accumulatingin condenser 6″ is also much higher than in the previous stages. Thecondensate 7 accumulating in 6″ goes to collection tank 24. The amountof monomers 1 and 2 in the cleavage products has become so low that aseparation is no longer worthwhile and, for this reason, everything iscondensed in condenser 8″. The vapors condensed in condenser 22 fromcompression 21 are supplied as compressor condensate 22 to thecollection tank 26. The decision is made on the remaining amount ofcondensate 9′ after analysis in quality control (QC) whether it canstill be supplied to rectification 8 to increase the yield of monomers 1and 2 and cleavage products 10, or is taken to collection tank 25 forspecial treatment according to FIG. 2.

The products collected in the tanks 24 to 28 are the results of a coarsefractionation or preliminary fractionation by selective choosing of thereaction and/or condensation conditions. They represent the first stageof an overall process that leads to an optimal and economic utilizationof the monomers and cleavage products. The product from collection tank24 has a high concentration of by-products, such as, for example,trisphenols, polymeric isopropenylphenols, dihydroxyindanes,dihydroxyspirobisindanes, alkyldistilbestrols, and polyhydroxyaryls, andonly a minor effort is required to recover reusable products such as themonomers or cleavage product. This is achieved by another rectification31 to increase the yield of monomers and cleavage products, before thehigh-boiling and discolored bottom product 32 is subjected to a thermalrecovery. Or otherwise disposed of. The content of monomers 1 and 2 andlow-boiling cleavage product 10 is less than 1% by weight in theoutgoing product 32.

Vapors 33 containing useful materials go to the top and proceed torectification 34, where they are rectified with product 9 and/or 9′ thatstems from the middle fraction collected in collection tank 25. In thiscase, a fraction each of monomers 1 and 2 is obtained from the lowertrays; after passing quality control (QC), these are again taken eitherdirectly after purification by crystallization 41, 43 and/or zone-meltprocess 42, 44 as pure monomer 1 and/or 2 to the first stage withreactor 4. The bottoms of stage 34 contain only high boilers, whichproceed to incineration 32.

Product 35 escaping to the top of column 34 is rich in low-boilingcleavage products and enters the middle portion of column 36, into whichthe main portion of the cleavage product from collection tanks 27 and 28is also introduced. Pure cleavage product 37, passing to the top, isrecovered, which after condensation 39 is returned via return tank 40and quality control (QC) either to column 36 or as the end product 45 isused for further reuse, for example, for the preparation of acid estersor of bisphenol A with acetone. The recovered product 45 meets thequality criteria of a product from the monomer synthesis. Qualitycontrol (QC) decides about the remaining bottom product 38 that iseither again allocated to column 34 or to incineration 32 as waste.

LIST OF REFERENCE CHARACTERS

-   1 monomer A diphenyl carbonate-   2 monomer B bisphenol A-   3 catalyst-   4 first reactor stage (first reaction of monomers)-   5 vapors (cleavage products, oligomers, monomers)-   6 high-boiler condensation first stage-   6′ high-boiler condensation second stage-   6″ high-boiler condensation third stage-   7 condensate (highest boiling points)-   8 rectification first stage-   8′ condensation second stage-   8″ condensation third stage-   9 middle condensate fraction-   9′ middle condensate fraction, not suitable for return to 8-   10 condensate (lowest boiling point), cleavage product condensate of    monomer A and/or monomer B-   11 condenser-   12 compressor (1^(st) pressure stage, highest pressure)-   13 product with a short chain length (1 to 10)-   14 second reactor stage (precondensation, medium chain length)-   15 vapors (cleavage products, oligomers, monomers)-   16 compressor (2^(nd) pressure stage, reduced pressure)-   17 gas cooler/compressor-condenser-   18 product with a medium chain length (5 to 50)-   19 third reactor stage (polycondensation, long chain length)-   20 vapors (cleavage products, oligomers, monomers)-   21 compressor (3^(rd) pressure stage, lowest pressure)-   22 gas cooler/compressor-condenser-   23 end product chain length (50 to 300)-   24 collection tank condensate highest boilers-   25 collection tank condensate high boilers-   26 collection tank compressor condensate, lowest pressure-   27 collection tank compressor condensate, highest pressure-   28 condensate collection tank compressor condensate, 1^(st)    compressor stage-   condenser, 1^(st) compressor stage-   30 bottom product rectification-   31 evaporator for products with highest boiling point-   32 waste for incineration-   33 evaporation product-   34 rectification for products with middle boiling point-   35 evaporation product-   36 cleavage product rectification-   37 pure cleavage product for further processing-   38 bottom product, after analysis waste 32 or back to 34-   39 cleavage product cooler-   40 collection tank cleavage products-   41 fractional crystallization monomer 1-   42 zone-melt purification method monomer 1-   43 fractional crystallization monomer 2-   44 zone-melt purification method monomer 2-   45 pure cleavage product for further reaction-   QC analysis site, quality control, and branching

1. A method of treating vapors and condensates that are produced duringthe production of polycondensates from bisphenols or polyhydric phenolsby esterification of acids and/or transesterification with alkyl and/oraryl esters of at least divalent organic or inorganic acids in amultiple stage polymerization with progressing vacuum wherein thepreparation of the vapors and condensates from each stage occurs inseveral succeeding cascaded condensation steps and distillation columnswith respective attached condensers, the dew point and pressure beingestablished in each condenser such that in each stage specific monomers,oligomers, or decomposition and rearrangement products are separated andcollected separately.
 2. The method according to claim 1 wherein themethod is used for treating vapors and condensates that are producedduring the production of polymers or copolymers based on diphenols,bisphenols, or phenyl esters.
 3. The method according to claim 1 whereinfor the condensation of the distillate recovered in a preparation step,the dew point, temperature, and pressure are selected such that in atleast one condenser predominantly the cleavage products or the monomersarising in the esterification and/or transesterification, are separatedand collected in a special collection tank.
 4. The method according toclaim 1 wherein for the condensation of the distillate recovered in apreparation step, the dew point, temperature, and pressure are selectedsuch that in at least one condenser predominantly the oligomers arisingin the preliminary condensation, are separated and collected in aspecial collection tank.
 5. The method according to claim 1 wherein forthe condensation of the distillate recovered in a preparation step, thedew point, temperature, and pressure are selected such that in at leastone condenser predominantly the reaction products arising during thepolycondensation by thermal decomposition or rearrangement are separatedand collected in a special collection tank.
 6. The method according toclaim 1 wherein the vapors and condensates are rectified in at least onedistillation stage.
 7. The method according to claim 1 wherein theproduct mixtures collected in the collection tanks are analyzed and thenpurified by at least one fine distillation and/or at least onecrystallization and supplied taken to the polycondensation.
 8. Themethod according to claim 1 through 6 wherein waste products areincinerated.